Ripening-related changes in cell wall polysaccharides of strawberry cortical and pith tissues

Abstract

Cell wall composition of cortical and pith tissues of strawberry (Fragaria x ananassa Duchesne cv. Yolo) was investigated at three stages of ripeness: unripe, partially ripe and ripe. Cell walls from these tissues, isolated using Hepes-buffered phenol (pH 6.5), were sequentially extracted with aqueous 0.1 M Hepes buffer, 0.05 M CDTA, 0.05 M Na2CO3, 1 M KOH and 4 M KOH. The uronic acids, neutral sugars and cellulose contents in these fractions were estimated colorimetrically, while monosaccharide composition of the neutral noncellulosic polysaccharides was determined by gas chromatography. During ripening, total cell wall polysaccharides (CWP) of cortical tissues decreased more (1.7 to 1.0 g/100 g frozen tissue) than that of the pith tissues (1.5 to 1.2 g/100 g frozen tissue). Concomitantly, cortical CWP showed greater changes in the extent of solubilisation compared to that of the pith—this appeared to be due to highly soluble pectic substances and the disappearance of highly branched pectic material. For pith, changes in CWP appeared to be associated with a marked decrease in non-esterified pectins. Monosaccharide composition of both tissue types indicated that losses in highly branched pectic substances were mainly due to a decrease in arabinose residues. By comparison, the cell walls of both tissue types exhibited only small decreases in hemicellulose during ripening. Cellulose appeared to be a minor cell wall component of strawberry fruit and its amount remained relatively constant throughout ripening.